Automotive lead-acid batteries have been used generally for starting engines and supplying power to electric components. In recent years, for environmental preservation and fuel cost improvement, idling stop and start of stopping an engine during temporary stopping of a vehicle and re-starting the engine upon starting (hereinafter referred to as ISS) has been introduced. Since engine starting and stopping are repeated frequently in ISS, the number of high rate discharge cycles increases in batteries upon starring the engine, and discharge load is increased being coupled with the use of electric components. A Battery has been charged by constant voltage charging as usual by an alternator, and a setting value for the alternator voltage has been lowered in recent years with an aim of reducing the decrease of electrolyte due to electrolyzing water during charging. In addition to the low charge voltage, a system of “controlling charging by the alternator during running in accordance with the running state of a vehicle and a charged state of a battery thereby decreasing the engine load, improving the fuel cost, and decreasing CO2”, which is referred to as a power generation control system, has also been adopted in recent years. It can be said that such a system provides a circumstance where the battery is less charged and fully charged state is less obtained. Under such a working condition, the battery is not charged fully and often used in an over-discharged state.
When the battery is not charged fully and a poor charged state continues, this causes a phenomenon that lead sulfate as an inert discharging product is accumulated on plates (sulfation). It has been known that the active material is less reduced (less charged) and the battery performance is deteriorated under such a situation. Further, when the fully charged state is less obtainable, a stratification phenomenon occurs to cause difference in the concentration of a diluted sulfuric acid as an electrolyte between the upper portion and the lower portion of plates of the battery. In this case, the concentration of the diluted sulfuric acid increases in the lower portion of the plates and sulfation arises. Accordingly, the reactivity is lowered in the lower portion of the plates and reaction is concentrated only to the upper portion of the plates. As a result, degradation such as weakening of bonding between the active materials proceeds in which the active material is defoliated from grid in the upper portion of the plate to lower the battery performance and shorten the life. As described above, since recent batteries have been used in an poor charged state, it has been demanded for the improvement of charge acceptance.
As means for improving the charge acceptance of the battery, Patent documents 1 and 2 disclose a technique of adding an additive to a negative active material.
The Patent document 1 discloses a technique of simultaneously adding a synthetic lignin having a molecular weight of about 17,000 to 20,000 (condensate of bisphenols and amino benzene sulfonic acid) and a carbon black having a specific surface of 150 to 300 m2/g obtained from heavy oil as a starting material to a negative active material of a lead-acid battery in order to improve the charge acceptance.
The Patent document 2 discloses a technique, for obtaining a valve regulated lead-acid battery of a long life, of subjecting acetylene black or natural flake graphite to hydrogen reduction, preparing a paste type negative plate in which the reduced acetylene black or natural flake graphite is incorporated in an active material layer, and using the paste type negative plate.